Current photovoltaic technology development has grown rapidly in the thin-film sector due to the commercial potential of thin films to be cost competitive with present energy-generating technologies in the near future. Thin-film solar cells demonstrate a large improvement in power generated per cost compared to similar technologies, in large part, due to their significant reduction in material consumption. In traditional bulk solar cells, efficiency increases were obtained by increasing the thickness of the solar absorbing layer. In thin-film solar cells, material reduction is enabled by improved optical and electrical properties including light scattering, light trapping and increased electron-hole collection efficiency.
Thin-film devices, however, still require many layers of materials to increase the absorption of photons and therefore increase the solar cell efficiency. In simple form, for example, amorphous silicon (a-Si) solar cells require first a layer of a metal electrode, then subsequently, a transparent conductive oxide (TCO) layer, an a-Si absorbing layer, another TCO layer, a scattering layer, and finally an antireflective (AR) coating as shown in FIG. 1. These layers are on the order of 10s to 100s of nanometers thick. High efficiency solar cells require many more layers and are stacked with much greater complexity.
As discussed above, TCOs are commonly used as electrodes both above and below the solar absorbing region of thin film solar cells (TFSCs). The TCOs are typically either a mixture of metal oxides (e.g., In2O3/SnO2) or a metal oxide containing substitutions of a dopant into a lattice site. In terms of deposition area, SnO2 doped with F or Sb is the most widely used transparent conductive oxide (TCO) material for many energy generation and electronic applications due to its low cost. Currently, high quality SnO2 requires a processing temperature of ˜450° C., which limits its applications in amorphous Si (a-Si) and copper indium gallium selenide (CCIGS) TFSCs.
All solar cells use photon absorption to generate electron-hole pairs, which, in turn, generate an electrical current. Photons in materials can be transmitted, reflected or absorbed. To maximize photon absorption it is necessary to reduce transmittance and reflection. The solar absorbing layer is responsible for the generation of electron-hole pairs; the remaining layers are designed to enhance the absorbing layer by techniques such as light scattering, light trapping, and electron-hole collection and transport.
Electrode pairs, one positive and one negative, are used to facilitate electron hole collection and transport; the higher the electrical conductivity of the individual electrodes, the greater the energy conversion efficiency. The metal electrode is typically a high conductivity metal, such as Al. Most metal electrodes are thick enough that they act as light reflectors for solar wavelengths, eliminating most transmission losses. The scattering layer contains nano-features and materials of different refractive indices to scatter light, increasing the mean path length of light, and therefore increasing the probability that photons will be absorbed in the absorbing layer. The two TCO layers are also used as electrodes: the bottom TCO acts as a conducting buffer between the solar absorbing layer and the metal electrode while the top TCO comprises the entire top electrode to allow light to transmit to the solar absorbing layer. The TCO electrodes are almost always inorganic metal oxide (MO) layers due to their transparency and conductivity, as well as their chemical and structural stability. Sometimes one or both of the TCO layers will be textured so as to scatter light in lieu of the scattering layer. Finally, the antireflective (AR) coating causes destructive interference of reflected light thereby reducing reflection losses. Good AR coatings, with multiple layers of precisely controlled material thicknesses, can cut reflection from 25%-45% to <5% (Alves, Lucas et al. High-Efficiency Solar Coatings. Depositions Sciences, Inc. Solar Novus Today, available at www.depsci.com/Documents/NewsRoom/DSI%20High-Efficiency%20Solar%20Coatings.pdf), greatly increasing light absorption. These three components: electrodes for electron-hole collection and transport, a scattering layer/microstructures for light trapping, and an antireflection coating, are used to maximize light absorption and energy generation in solar cells.
While these components have desirable properties for solar cells, current technology has some serious drawbacks. High conductivity electrodes are necessary to allow current flow from carrier generation; unfortunately, some of the standard TCOs used today contain relatively rare and expensive elements, such as indium in indium tin oxide (ITO). Textured TCO layers can enhance light trapping in the absorbing layer, yet at the cost of increasing surface recombination and reducing carrier collection efficiency. Finally, in an ideal situation, a light scattering layer should be directly adjacent to the active absorbing layer to maximize the scattering effect. In reality, however, light scattering layers are usually separated from the absorbing layer by the TCO layer due to electrical considerations. This greatly compromises the light trapping efficiency.